The present invention relates to foam breakers, and in particular to foam breakers for gas/liquid reactors (fermentors) and to reactors having such foam breakers.
Gas/liquid reactors are used, inter alia, in connection with the aerobic fermentation growth of micro-organisms. Among the most important tasks in this type of application are the need to create the largest possible interface between the gas phase and the bio-suspension liquid phase, the periodic renewal of this interface to eliminate diffusion blockages, and the uniform distribution of the micro-organism in the bio-suspension (nutrient) in order to avoid local enrichment and separation.
Ideally, the gas phase and the liquid nutrient phase should be absolutely uniformly mixed, i.e., should be completely uniformly distributed throughout. In most fermentation reactors this ideal condition cannot be attained. What happens instead is the formation of a liquid phase which is well mixed with the gaseous phase (usually aerated) and above which a layer of poorly mixed foam is formed. The formation of foam may be so strong that only 30-40% of the reactor volume can be filled with liquid, the rest being taken up by the foam. This foam formation is undesirable, because micro-organisms tend to conglomerate in the poorly mixed foam layer, with the result that local separations take place. Furthermore, the removal of waste gas (e.g., CO.sub.2) which develops during the fermentation, is made more difficult by the presence of the foam. And finally, a large proportion of the bio-suspension may be lost due to the formation and overflowing of the foam, unless special steps are taken to prevent this.
The problems outlined above have been recognized in the art for some time and the solution that has been proposed is the mechanical foam breaker, sometimes called a foam separator. However, the prior-art foam breakers are all fairly complicated from a structural viewpoint and are therefore expensive.
According to one proposal, made in German Pat. No. 2,518,082, conical members are mounted in the reactor head in front of the outlet opening for the waste gas; these are rotated at high speed, causing the surrounding foam to be flung off the conical members and the foam layer thereby to be destroyed, so that the waste gas can escape through its outlet opening. The mixing of the gaseous and liquid phases is accomplished or enhanced in fermentors of the type in question by means of paddles or similar instrumentalities mounted on a rotating shaft. For the foam breaker to operate effectively, the conical members must rotate at a much higher speed than this shaft, which means that the shaft and the foam breaker each require a separate drive. This raises the overall cost and energy requirements and, of course, increases the chances of possible malfunction. Particularly in the case of smaller reactors up to, say, about 20 liter volume this can be bothersome.
According to another proposal (Rehm, Einfuhrung in die industrielle Mikrobiologie, Springer-Verlag 1971, page 111) the foam layer is drawn off from the reactor and fed into an external foam breaker where the foam is separated into its liquid and gaseous constituents. The liquid component is continuously recirculated into the reactor. In this proposal the time between the formation and subsequent destruction of each foam bubble is relatively long, which means that the dwell time of individual parts of the bio-suspension in the poorly mixed foam layer is correspondingly long, so that local separations occur.
Still another proposal (German Published Application DE-OS No. 2,719,112) combines the earlier-mentioned conical rotary members with a tube system which operates as a gas exchanger. Again, the proposal works, but it is structurally relatively complicated and therefore only of limited applicability.